Overset Grid Assembly Research Articles

Exact and efficient wall distance pre-classification overset grid assembly for unstructured grids

In this paper, we present a pre-classification overset grid assembly (OGA) based on the wall distance criterion. In contrast to the conventional OGA, where donor search is preceded by point classification, the proposed methodology first performs point classification based on the wall distance criterion, and then performs donor search on the reduced set of cells. To robustly profile the given grid components, a wall distance evaluation is performed exactly by considering the full geometry of the discretized surface while maintaining efficiency. The efficiency of the algorithm is achieved by redefining the methodology into a bounded form and establishing a conservative correlation between the vertex-based and element-based distances through a rigorous examination of geometric relations. From these observations, wall distance calculation, which requires a tremendous amount of computation, is divided into sequential steps of vertex-based approximate calculation and element-based exact calculation, dramatically reducing the computation load. To better adapt to parallel environments, load balancing is presented, and additionally, some voxelization techniques are developed to efficiently process the required geometric data. Numerous test cases are analyzed to demonstrate the efficiency and robustness of the proposed algorithm in various situations. The proposed algorithm consistently outperforms the conventional OGA, and also proves its robustness in the presence of high proximity objects, which highlights its suitability for large-scale multi-body problems.

A new overset grid assembly strategy for dynamic grid systems

In overset grid assembly (OGA), the efficiency of assembly has been a major concern. In this paper, we propose an OGA strategy focuses on the re-assembly process in dynamic grid systems. Based on the recognition and expansion process of the hole boundary, we define a new set of cells for continuing OGA step. In this method, the number of cells participating in the new OGA process constitutes only a small portion of the primary grid system, leading to an acceleration in assembly and wall distance calculation. Not limited to the dynamic grid system, the static OGA is also improved based on the proposed method, without the need for additional mesh and only need minor changes to the program. We investigate cases including static and dynamic grid systems to verify the efficacy of the proposed method. The results confirm that the new method reduces the time consumption by 50% compared to the original method, particularly in the improvement of wall distance calculation.

Local Inverse Mapping Implicit Hole-Cutting Method for Structured Cartesian Overset Grid Assembly

An automatic hole-cutting method is proposed to search donor cells between a structured Cartesian mesh and an overlapping body-fitted mesh. The main flow is simulated on the structured Cartesian mesh and the viscous flow near the solid boundary is simulated on the body-fitted mesh. Through the spatial interpolation of flux, the surface boundary information on the body-fitted mesh is transferred to the Cartesian mesh nodes near the surface. Cartesian mesh box near a body-fitted mesh cell is selected as a local inverse map. The Cartesian nodes located inside the donor cells are marked by the relative coordinate transformation, so that all Cartesian nodes can be classified and the hole boundaries are implicitly cut. This hole-cutting process for overset grid assembly is called Local Inverse Mapping (LIM) method. In the LIM method, spatial interpolation of flux is carried out synchronously with the marking of Cartesian nodes. The LIM method is combined with the in-house finite-difference solver to simulate the unsteady flow field of moving bodies. The numerical results show that the LIM method can accurately mark the Cartesian hole boundary nodes, the search efficiency of donor cells is high, and the result of spatial interpolation is accurate. The calculation time of overset grid assembly (OGA) can be less than 3% of the total simulation time.

Enhancement on parallel wall distance calculation methodology for partitioned unstructured grid

AbstractMinimum distance to a solid wall is a primary parameter in turbulence models and overset grid assembly for computational fluid dynamics. In present work, a parallel advancing front method is proposed based on partitioned unstructured grids for the sake of further efficient wall distance computation. In order to overcome the inherent problem of conventional advancing front method in parallel environment, a novel “advancing twice and rippling once” strategy is developed to compute wall distance efficiently and further recover the accuracy. Significantly, the established framework is of modular nature to be easily extended to overset grid system for complex multi‐body configurations. The performance of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency and accuracy. Subsequently, further case studies are performed to examine its capability to deal with complex engineering applications such as a full transportation, a wing‐store configuration, a helicopter and a F‐16 fighter with onboard payloads. Results show that the proposed advancing front methodology is in practice able to solve extremely complex geometries with both convex and concave shapes with high efficiency and robustness.

Enhancement on parallel unstructured overset grid method for complex aerospace engineering applications

In the present study, an efficient overset grid method by means of parallel implicit hole-cutting is proposed for the sake of simulating unsteady flows in aerospace engineering involving multiple bodies in relative movement. In view of the degraded computational efficiency and robustness for conventional overset grid assembly, several innovative techniques are developed within the overset grid assembly process, viz., a bookkeeping alternative digital tree method to speed up the donor-cell searching, a fast parallel advancing front algorithm to accelerate the wall-distance calculation and a message-passing strategy with efficient information communication and lower storage expenditure within distributed computational architecture. The contribution of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency. Subsequently, the overset grid method is embedded into an in-house programed URANS solver to examine its capability in predicting the flow field of complex applications such as helicopter, store separation and component deploying. Results show that the developed overset grid methodology is, in practice, able to resolve the aerodynamic characteristics of complex aerospace engineering with a high-fidelity flow topology and accuracy.

Highly Efficient Wall-Distance-Based Parallel Unstructured Overset Grid Assembly

The overset grid assembly (OGA) method is an effective computational fluid dynamics technique for the simulation of flows with complex geometries or relative moving parts. Using OGA, the effort for mesh generation is significantly reduced by generating the grid for different parts and then assembling them to form the computational grid. New methods have been developed that are generally divided into implicit and explicit methods in which a donor cell search for the defined query points is often time-consuming. Besides, the parallelization of OGA is a complex task that is not currently supported by many software platforms. In this paper, a new OGA approach is developed based on the wall distance and a user-defined reference length. The proposed approach is relatively more convenient to implement and more robust among the existing methods. The number of query points defined by this method is also significantly reduced, resulting in a higher speed of performing the overall donor search process. Validity tests are conducted, and the results confirm the effectiveness of the proposed method.

A robust overset assembly method for multiple overlapping bodies

SummaryA robust overset assembly method allowing for multiple overlapping bodies is presented. This method extends the overset assembly method to deal with the interference between objects and prevents the failure of the search for donor cells due to the complex shape of the models. The donor cells are searched before identifying the boundary of the chimera interpolation. By selecting interpolation cells only in areas with donor cells, the final overset interpolation boundary cells can always find their donor cells, which prevents orphan points and achieves excellent robustness. First, the implementation of the proposed assembly method is described in detail. Subsequently, the usability and efficiency of the method are verified using several cases. Finally, the integration strategy of the computational fluid dynamics solver based on overset interpolation with the modification of the boundary type is described and verified using practical cases with overlapping bodies. The results demonstrate the applicability of the proposed overset grid assembly method.

Performance of Overset Mesh in Modeling the Wake of Sharp-Edge Bodies

The wake dynamics of sharp-edge rigid panels is examined using Overset Grid Assembly (OGA) utilized in OpenFOAM, an open-source platform. The OGA method is an efficient solution technique based on overlap of a single or multiple moving grids on a stationary background grid. Five test cases for a stationary panel at different angle of attack are compared with available computational data, which show a good agreement in predicting global flow variables, such as mean drag. The models also provided accurate results in predicting the main flow features and structures. The flow past a pitching square panel is also investigated at two Reynolds numbers. The study of surface pressure distribution and shear forces acting on the panel suggests that a higher streamwise pressure gradient exists for the high Reynolds number case, which leads to an increase in lift, whereas the highly viscous effects at low Reynolds number lead to an increased drag production. The wake visualizations for the stationary and pitching motion cases show that the vortex shedding and wake characteristics are captured accurately using the OGA method.

Overset grid assembly approach for scalable computational fluid dynamics with body motions

A methodology for the decomposition of the overset grid assembly problem is presented and evaluated. The method is based on identifying grid subsets of a computational domain for which each domain connectivity is calculated individually, and combined at run time to obtain the final complete solution. The proposed method is a framework for the partition of the problem based on the characteristics of the simulation and relies on the user's knowledge of the case, avoiding some of the pitfalls of automatic methods. The methodology was implemented in a computational fluid dynamics solver, allowing the scalable computation of very large simulations of moving bodies, with efficient use of the computational resources. Several examples illustrating the method and the performance improvements with respect to the standard assembly approach are presented.

High-resolution multi-code implementation of unsteady Navier–Stokes flow solver based on paralleled overset adaptive mesh refinement and high-order low-dissipation hybrid schemes

ABSTRACTThe low-dissipation high-order accurate hybrid up-winding/central scheme based on fifth-order weighted essentially non-oscillatory (WENO) and sixth-order central schemes, along with the Spalart--Allmaras (SA)-based delayed detached eddy simulation (DDES) turbulence model, and the flow feature-based adaptive mesh refinement (AMR), are implemented into a dual-mesh overset grid infrastructure with parallel computing capabilities, for the purpose of simulating vortex-dominated unsteady detached wake flows with high spatial resolutions. The overset grid assembly (OGA) process based on collection detection theory and implicit hole-cutting algorithm achieves an automatic coupling for the near-body and off-body solvers, and the error-and-try method is used for obtaining a globally balanced load distribution among the composed multiple codes. The results of flows over high Reynolds cylinder and two-bladed helicopter rotor show that the combination of high-order hybrid scheme, advanced turbulence model, and overset adaptive mesh refinement can effectively enhance the spatial resolution for the simulation of turbulent wake eddies.

An Overset Grid Method for Fluid-Structure Interaction

An overset grid methodology is developed for the fully coupled analysis of fluid-structure interaction (FSI) problems. The overset grid approach alleviates some of the computational geometry difficulties traditionally associated with Arbitrary-Lagrangian-Eulerian (ALE) based, moving mesh methods for FSI. Our partitioned solution algorithm uses separate solvers for the fluid (finite volume method) and the structure (finite element method), with mesh motion computed only on a subset of component grids of our overset grid assembly. Our results indicate a significant reduction in computational cost for the mesh motion, and element quality is improved. Numerical studies of the benchmark test demonstrate the benefits of our overset mesh method over traditional approaches.

Robust and efficient overset grid assembly for partitioned unstructured meshes

This paper presents a method to perform efficient and automated Overset Grid Assembly (OGA) on a system of overlapping unstructured meshes in a parallel computing environment where all meshes are partitioned into multiple mesh-blocks and processed on multiple cores. The main task of the overset grid assembler is to identify, in parallel, among all points in the overlapping mesh system, at which points the flow solution should be computed (field points), interpolated (receptor points), or ignored (hole points). Point containment search or donor search, an algorithm to efficiently determine the cell that contains a given point, is the core procedure necessary for accomplishing this task. Donor search is particularly challenging for partitioned unstructured meshes because of the complex irregular boundaries that are often created during partitioning.Another challenge arises because of the large variation in the type of mesh-block overlap and the resulting large load imbalance on multiple processors. Desirable traits for the grid assembly method are efficiency (requiring only a small fraction of the solver time), robustness (correct identification of all point types), and full automation (no user input required other than the mesh system). Additionally, the method should be scalable, which is an important challenge due to the inherent load imbalance. This paper describes a fully-automated grid assembly method, which can use two different donor search algorithms. One is based on the use of auxiliary grids and Exact Inverse Maps (EIM), and the other is based on the use of Alternating Digital Trees (ADT). The EIM method is demonstrated to be more efficient than the ADT method, while retaining robustness. An adaptive load re-balance algorithm is also designed and implemented, which considerably improves the scalability of the method.

Enhancement and Application of Overset Grid Assembly

In this article, the overset grid assembly method is improved to efficiently solve several critical problems that occur when applying overset grids to the complicated geometries and moving body. First, instead of using the two-step searching (i.e. cut and paste), a single-step searching method based on the grid cell size is proposed to modify holes and optimize the grid overlapping automatically. Second, discrepancies between the wall surface representations, where the grids overlap, are handled by introducing the wall's normal directions to the hole-map and projecting the interpolated points on the recipient mesh into the donor mesh. Finally, the dynamic overset method is modified to address the complex moving body problem. At every time step, the initial hole surface of the previous time step is dynamically adjusted to accomplish hole cutting and avoid the time consuming hole-map procedure. Numerical experiments show that the enhanced overset grid assembly method obtains satisfactory results.

Parallel domain connectivity algorithm for unsteady flow computations using overlapping and adaptive grids

This paper describes the algorithms and functionality of a new module developed to support overset grid assembly associated with performing time-dependent and adaptive moving body calculations of external aerodynamic flows using a multi-solver paradigm (i.e. different CFD solvers in different parts of the computational domain). We use the term “domain connectivity” in this paper to denote all the procedures that are involved in an overset grid assembly, and the module developed is referred henceforth as the domain-connectivity module. The domain-connectivity module coordinates the data transfer between different solvers applied in different parts of the computational domain – body fitted structured or unstructured to capture viscous near-wall effects, and Cartesian adaptive mesh refinement to capture effects away from the wall. The execution of the CFD solvers and the domain-connectivity module are orchestrated by a Python-based computational infrastructure. The domain-connectivity module is fully parallel and performs all its operations (identification of grid overlaps and determination of data interpolation strategy) on the partitioned grid data. In addition, the domain connectivity procedures are completely automated such that no user intervention or manual input is necessary. The capabilities and performance of the package are presented for several test problems, including flow over a NACA 0015 wing and an AGARD A2 slotted airfoil, hover simulation of a scaled V-22 rotor, and dynamic simulation of a UH-60A rotor in forward flight. A modification to the algorithm for improved domain connectivity solutions in problems with tight tolerances as well as heterogeneous grid clustering is also presented.